# The Changing Effects of Petco Park

Jeff Sullivan’s recent enjoyable trot through San Diego Padres statistics and history led to a number of commentors thinking about San Diego’s park factors. The Padres changed the outfield dimensions of Petco Park in the off-season, and since park factors are backwards looking and rely on multiple years of data, changing dimensions can throw a bit of a monkey wrench into the calculations. So, it’s possible that our park factors are now somewhat behind the times, and we need to keep this in mind when looking at the park adjusted numbers (such as wRC+, ERA-/FIP-/xFIP-, WAR, etc…) for San Diego players, both hitters and pitchers.

It’s not quite so simple as noting that the changing dimensions have made the old park factors useless, however. Moving in the fences helps home runs, yes. This is undeniable. But it also can decrease triples and doubles, as well as effect the more odd elements of park factors, such as walk-rates, strikeout rates and pop-up rates.

It’s too early in the season to construct terribly useful park factors for the new dimensions, but we can do some harmless back-of-the-napkin mathematics to at least determine if the recent numbers suggest at least the early signs of serious run environment changes.

Below are the extra base hit rates (per PA) for Petco Park stretching back to 2005:

Doubles and triples are down, and home runs are way up. For greater specificity, we can see those same numbers in table form:

Season 1B% 2B% 3B% HR%
2006 15% 3.87% 0.72% 2.46%
2007 14% 4.18% 0.63% 2.39%
2008 15% 3.81% 0.49% 2.15%
2009 13% 3.51% 0.53% 2.02%
2010 15% 3.36% 0.47% 1.96%
2011 14% 3.77% 0.78% 1.56%
2012 15% 4.48% 0.84% 1.57%
2013 15% 3.82% 0.37% 2.40%

Does anyone else find it curious the 2013 doubles rate compares favorably to the same rate from 2008 through 2010? And the home run rate matches the unusual 2006 blip? This is why most (good) park factors include multiple years — in an effort to avoid catching weird blips — and include adjustments to reflect league-wide run environments. Odds are, some of the changes here may be reflections of the Padres personnel and the ever-morphing strengths of the NL West as much they reflect the effects of the park itself.

So has Petco Park changed appreciably this season? We can’t say. What we can say: Doubles have decreased 0.42% since the 2005-2012 period, triples have decreased 0.22% and homers have increased 0.72%.

If we convert that to run values (using 2013 adjusted constants, i.e. divide the constants by the wOBA scale):

#### Debit

2B: 0.0042 x (1.262/1.262) = 0.42% fewer runs per PA
3B: 0.0022 x (1.608/1.262) = 0.28% fewer runs per PA

#### Credit

HR: 0.0072 x (2.080/1.262) = 1.19% more runs per PA

That’s a 0.49% increase run value per PA. In (3 x 9 PA) 27 PA of a game, that’s an increase of 0.135 runs. In about 7 games, they are scoring an extra run; that’s an extra run per week. So we can say this: More game-context-neutral offense has thus far occurred at Petco Park.

I say that so awkwardly on purpose. I do not want to suggest these home park numbers will continue on their present pace without any further fluctuation. What he have is 36 games, and that’s all we can really speak about with any authority.

Another question we may also want to address: how has the Padres run scoring / run prevention changed? Well, with the Padres hitters, we see the same pattern of decreased doubles and triples, increased homers:

And though the doubles rate decreased well beneath 2013 levels, it nearly equals the 2006-2011 levels. The homer rate is considerably higher than the preceding years, but is not much above the Padres’ late “steroid era” years.

And with the pitchers, the DIPS numbers are steady with the essential, weighty exception of their home run rate (NOTE: I’ve removed IBB from the BB-rate):

That is a 1.97% HR-rate ballooning to a 3.09% HR-rate. It’s fair to say the pitching staff is a collection of non-studdish hurlers, but regardless of their mediocrity, they are maintaining nearly identical strikeout rates and walk rates with respect to recent rosters.

Is Petco Park haunted by its former dimensions? Yes, and we should keep that in mind when looking at the Padres park adjusted numbers. The Padres pitchers might not be quite as bad as those numbers make them look, since Petco probably isn’t quite as helpful to hurlers as it used to be. However we will need more time before we can measure the magnitude of these changes over the long term, and it is important to remember that changing the dimensions doesn’t turn a pitcher’s paradise into a hitte’s haven overnight.

We hoped you liked reading The Changing Effects of Petco Park by Bradley Woodrum!

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Guest

It’s also worth point out that in years past Petco tends allow more runs after July 1, when the marine layer isn’t playing defense.

I believe those numbers used for previous years reflect an entire season. Overall the offense has increased in Petco and I would expect that to raise as the weather heats up.

Member
Richie

What do you mean by “marine layer” “playing defense”? And can you show the numbers supporting this?

I’d always figured West Coast parks played closer to neutral in April, while the rest of the country was frigid. If this marine layer works that way in San Diego, might it also do so in Los Angeles/San Fran/Seattle?

Guest
The Humber Games

“If this marine layer works that way in San Diego, might it also do so in Los Angeles/San Fran/Seattle?”

It’s basically an inversion zone of humid air and humidity isn’t very good for home runs. I’m unsure of which of the west coast ballparks are most and least affected, but it can be a factor until midsummer depending on how hot it is.

Member

Pretty sure Seattle’s marine layer is the defensive equivalent of Roberto Clemente.

Guest
Baltar

I happen to have spent considerable time in all 4 major West Coast locations, primarily LA and SF. One thing I can say for certain: the hotter it is inland, the cooler it is on the coast. If the marine layer does make a difference, it should be most pronounced in July and August.
It’s great to see you back on FanGraphs, Bradley.

Guest
Natty Bunto

“humidity isnâ€™t very good for home runs”

Do we know this? The old saw about the “heavy, humid air” is false on its face. Water vapor is actually less dense than air, so balls travel further in humid air, all else equal. On the other hand, humid balls are heavier, and pitchers also can get a better grip on it when it’s not bone dry. I’ve not yet seen a definitive “reckoning” of these various contradictory effects. Has anyone else?

SI International, makers of the “Home Run Weather” app, seem to imply that humidity increases home run propensity:

http://projectprospect.com/article/2012/08/28/home-run-weather-analysis

BTW, I’ve downloaded this app and it’s at least a fun toy for planning fantasy pitching choices.

Member
Richie

What eventually comes down must first go up. Since the ball starts at ground level (basically), the more humid and heavy the air, the less high the ball gets in the first place. The less distance it then has to get back to the ground, hence the quicker it does.

I believe way back in the 80s Bill James cited a study showing this.

Guest
Natty Bunto

Richie, you missed my main point. Humid air is not “heavy” at all, it’s light – lighter (thinner) than dry air. This popular canard started because humid air feels “heavy” to humans because it’s oppressive feeling.

Thus, the effect of the air solely on the travel of the ball is that it causes it to go further. Of course, there are other effects of the humidity that run counter to this, as I alluded to. Another that I didn’t mention was that humidity makes the ball “mushier” and lowers the elasticity of the bat-ball energy transfer.

I am just wondering if anyone anywhere has ever “added up” all these contradictory effects to give us the bottom line.

Guest
MGL

I have done some research over the years on humidity versus HR rates and overall scoring.

The lighter air (less friction so greater speed and distance on batted balls) appears to be balanced by “soggier and easier to grip balls” to produce an approximate net zero effect.

Now, if there is truly a “layer of humid air” somewhere above the ground, but not on the ground (and I have no idea if that is true – I am not a meteorologist, I only play one on TV), then I suspect that that would cause the ball to fly further, since there would be no offsetting soggier ball factor.

Also, I don’t know if actual condensed moisture in the air (like mist or fog, which is water condensed on dust particles I think) might create more friction and thus slow the ball down, as opposed to humid air, which is indeed lighter and speeds the ball up.

It is amazing how many people quote the “heavy air and thus the ball does not travel well” mantra, whenever humidity and baseball are spoken of. You hear it all the time on TV from the commentators.

What is often thought of as humid air slowing the ball down in coastal night games is actually cook weather and heavier sea level air. The heavy sea level air is just not noticed as much during the day when it is warmer and the lighting is better for the hitters…

Guest
MGL

And of course “cook” weather should have read “cool” weather. I did not mean nice, warm weather, as in “a good day for a barbecue…”

Guest
evo34

It’s mostly about temperature, which has the greatest impact on flyballs.. West coast temps. don’t peak until August. So there is a legitimate issue trying to use half a season to even guess at a park factor at a west coast park.